This application claims priority from provisional application Ser. No. 60/824,385 filed Sep. 1, 2006 and from Ser. No. 11/739,307 filed Apr. 24, 2007.
The present invention represents an improvement in the manufacture and structure of light emitting diodes that incorporate phosphors for purposes of color conversion, blending or both.
Light emitting diodes (LEDs) are a class of semiconductor devices that generate photons when a current is passed across a p-n junction. In their fundamental structure, light emitting diodes include at least one p-type layer and one n-type layer that together defined the junction. When current is injected across the junction, electrons and holes recombine and can create photons. In accordance with well-understood principles of electronics and physics, the wavelength (and thus the frequency) of the photons is based upon the energy change of the recombination. In turn, the energy is either defined or constrained by the bandgap of the semiconductor material i.e., the energy difference between the material's valence band and its conduction band.
As a result, the color emitted by an LED is largely defined by the material from which it is formed. Diodes formed of gallium arsenide (GaAs) and gallium phosphide (GaP) tend to emit photons in the lower energy red and yellow portions of the visible spectrum. Materials such as silicon carbide (SiC) and the Group III nitrides have larger bandgaps and thus can generate photons with greater energy that appear in the green, blue and violet portions of the visible spectrum as well as in the ultraviolet portions of the electromagnetic spectrum.
In some applications, an LED is more useful when its output is moderated or converted to a different color. In particular, as the availability of blue-emitting LEDs has greatly increased, the use of yellow-emitting phosphors that convert the blue photons has likewise increased. Specifically, the combination of the blue light emitted by the diode and the yellow light emitted by the phosphor can create white light. In turn, the availability of white light from solid-state sources provides the capability to incorporate them in a number of applications, particularly including illumination and as backlighting for color displays. In such devices (e.g., flat computer screens, personal digital assistants, and cell phones), the blue LED and yellow phosphor produce white light which is then distributed in some fashion to illuminate the color elements (often formed by liquid crystals, “LCDs”).
In the present application, the term “white light” is used in a general sense. Those familiar with the generation of colors and of color perception by the human eye will recognize that particular blends of frequencies can be defined as “white” for precise purposes. Although some of the diodes described herein can produce such precise output, the term “white” is used somewhat more broadly herein and includes light that different individuals or detectors would perceive as having a slight tint toward, for example, yellow or blue.
Many of the phosphors used in conjunction with LEDs are fine particles of fluorescent compositions that are positioned adjacent the LED chip. In the most typical example, the LED chip is mounted on a circuit board or other electrical connection (e.g., a lead frame) and is encapsulated by a substantially transparent material that forms a lens. Most typically, the lens is a polymer, very often an epoxy or a silicone. When a phosphor is incorporated, it is typically introduced as a suspension in the resin. The suspension is then applied to the LED and cured. The resulting package (chip, electrical leads, lens) is often referred to as an LED lamp.
Because the phosphor is made up of particles and because it is distributed in the resin, its physical position with respect to the LED chip can affect the LED's efficiency and output. When the distribution of the phosphor particles in the resin cannot be properly controlled, the resulting LED lamp may suffer from a less suitable output than its physical and electronic properties could otherwise provide.
As the use of blue LEDs in conjunction with yellow phosphors has increased, it has become evident that certain resins, including many epoxy resins, are less suitable for such higher energy devices because they are more easily prone to photochemical reactions (unfortunately mostly resulting in degradation) when exposed to blue and higher energy photons.
As another factor, adding phosphor particles to a polymer resin and then applying the phosphor-containing resin to the LED chip requires a series of manufacturing steps. Generally speaking, the resin has to be prepared in a liquid form so that it can be cast or otherwise formed into the desired package shape. Additionally, the phosphor has to be mechanically mixed with the resin in such liquid form so that it can be incorporated in the package. For practical manufacturing purposes, the resin should have a reasonable “pot life;” i.e., the time interval during which it can be worked before it starts to cure.
Accordingly, a need exists for combinations of phosphors and resins, and techniques for handling phosphors and resins, that enhances, rather than complicates, the process for manufacturing light emitting diode lamps.